Montoro made the comments during a brief attack on the economic policies of the UK territory which is home to 30,000 people.

Describing Gibraltar's business operating environment as "opaque", he said the territory was a "magnet" for businesses with "doubtful" operations.

Gibraltar's economic structure was costing Spain €1 billion ($1.3 million) a year in lost tax revenues, said Montoro, repeating a tax office estimate which Gibraltar authorities have dismissed as fanciful.  

Spain's finance minister also stressed that UK figures showed 21,670 companies operated out of the territory but only 11 percent of those paid tax. Many of those businesses have "one of no" employees, he added. 

Montoro went on to say Spain planned to investigate Gibraltar residents who owned properties in Spain.

"On the Rock there are more than 6,000 citizens who used our public services, but are registered as living in Gibraltar," Montoro said.

Spanish authorities were also carrying out checks on cars with Gibraltar number plates which belong to people living in Spain, he said.

Montoro's comments come in the wake of a report from  European Union's anti-fraud office Olaf which claimed the EU lost €700 million in tax revenues from 2009 to 2013 as a result of tobacco smuggling through Gibraltar.

Olaf's 400-page report is the result of investigations into a complaint concerning increased smuggling of cigarettes across the frontier between Gibraltar and Spain in the period 2009-2013, an OLAF spokesperson told The Local recently.

Spanish media outlets have said Gibraltar's cigarette imports tripled from 2006 to 2011, with 110 million packets coming into the territory in 2012 alone.

Much of this tobacco is thought to be smuggled into Spain.

There are also indications organized crime is involved in the trade, Olaf told The Local, calling for Spain and Gibraltar to tackle the problem.

Spanish authorities responded to the report — which was not made public — by saying they would investigate suspected illegal activity in the tiny UK territory.

Gibraltar's Chief Minister Fabian Picardo responded by saying Gibraltar was already clamping down on the illegal trade, and stressed it was willing to work with Spain to deal with the problem. 

In a statement, he quoted a UK government statement which stressed the presence of organized criminal gangs in Spain, which was "a huge concern for law-abiding citizens on both sides of the border".

Relations between London and Madrid became particularly strained last year after Gibraltar dropped 70 concrete blocks into the sea in July, in what its government said was an attempt to create an artificial reef.

The move had the effect of also blocking Spanish fishing boats from operating close to the airport runway, and Madrid responded by introducing stringent border checks.

Spain has repeatedly criticized Gibraltar for failing to clamp down on its imports, and many in Spain see the UK territory with 30,000 residents as a haven for smugglers and tax evaders.